Following treatment of rats for varying periods of time with the monoamine oxidase inhibitors, phenelzine, tranylcypromine, clorgyline or pargyline, changes in endogenous levels of norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) have been measured. In addition, regional changes in tyrosine hydroxylase, tryptophan hydroxylase and aromatic L amino acid decarboxylase were assessed. There are initial increases in whole brain NE, DA and 5-HT levels with a peak between 3-7 days, followed by a gradual decrease towards control levels over the next several weeks. Striatal tyrosine hydroxylase (TH) activity becomes significantly decreased from control levels by the third week of phenelzine and tranylcypromine treatment and remains so at six weeks. TH is unaffected by chronic pargyline or clorgyline treatment. Tryptophan hydroxylase (TpOH) is significantly increased in mesencephalic tegmentum at three and at six weeks of phenelzine treatment. Clorgyline and pargyline did not affect TpOH activity. Decarboxylase was unchanged by phenelzine treatment but increased by tranylcypromine. BIBLIOGRAPHIC REFERENCE: Skyler, Jay S., Rogol, Alan D., Lovenberg, Walter and Knazek, Richard A.: Characterization of growth hormone and prolactin produced by human pituitary in culture. Endocrinology 100: 283-291, 1977.